Sales of the bulbous Lexus SC 430, one of the vehicles that kick-started the current hardtop convertible fad in the U.S., have slowed so significantly that Toyota's luxury brand has reportedly canceled its replacement. The four-place, V8-powered, rear-wheel-drive vehicle never fit well into any specific niche, as it boasted neither the sportiness of the ragtop Jaguar XK and Porsche 911, nor the snob appeal of the BMW 6 Series. Through the first six months of this year, Lexus sold only 1,230 SC's